Revert of Move content's shell connections to the IO thread

content/public/test/mock_render_process_host.cc

Index: content/public/test/mock_render_process_host.cc
diff --git a/content/public/test/mock_render_process_host.cc b/content/public/test/mock_render_process_host.cc
index d6cac6f56a3f80ffd60adf51c117993a1505a8a2..4eed8a75fe59f4ba2bc60f05065e246f20ece953 100644
--- a/content/public/test/mock_render_process_host.cc
+++ b/content/public/test/mock_render_process_host.cc
@@ -269,6 +269,10 @@
   return remote_interfaces_.get();
 }
 
+shell::Connection* MockRenderProcessHost::GetChildConnection() {
+  return nullptr;
+}
+
 std::unique_ptr<base::SharedPersistentMemoryAllocator>
 MockRenderProcessHost::TakeMetricsAllocator() {
   return nullptr;